Temperature is a measure of the average energy of the motion of the atoms within an object. In most practical cases, it is only possible to measure this energy, or the temperature, indirectly usually by measuring other material properties which are known to be temperature dependent in a reproducible way.
Accurate temperature measurements can be achieved by placing a calibrated sensor, whose temperature-dependent material property is well characterised, in contact with the object or material of interest. Once the temperature of the sensor reaches that of the object, the indirect property can be used, via a calibration equation, to mathematically determine the object's temperature. Typical contact sensors are platinum resistance thermometers, thermocouples and thermistors.
Where contact measurements are not possible, other non-contact methods are available. For example, all objects above absolute zero emit electromagnetic radiation from their surfaces. This is primarily in the infrared range and for objects at or below approximately 1000 K (~700 °C) the radiation is not visible to human eyes. The peak wavelength and intensity of the emitted radiation is dependent on the temperature of the object. Non-contact, or radiation, thermometers are designed to detect the intensity of this radiation, and therefore determine the temperature of the object. More recently non-contact detector technology has developed towards creating multi-pixel arrays where many single spot readings are taken simultaneously and presented as pixels in an image. This is known as thermal imaging.
For calibration purposes, it is important to have environments which can be repeatedly set to have a known temperature. For a comparison calibration, a fluid bath or solid block is used. The bath or block is designed to be as uniform in temperature as possible so that the thermometer under test is in the same thermal environment as a reference thermometer, whose calibration is known. The temperature of the bath or block is then determined by the reference thermometer.
For higher accuracy calibration, a set of reference temperature 'fixed points' can be used. Temperature fixed-point cells have extremely reproducible behaviour and typically use either the melting point, freezing point or triple point (where the solid, liquid and vapour coexist) of pure elements. These fixed points have been studied internationally, and the value of the temperature of each agreed and defined in the International Temperature Scale of 1990 (ITS-90). The ITS-90 can be used in the temperature range above 0.65 K and is the agreed temperature scale in use around the world.
NPL has a world-leading team of researchers working on improving existing temperature measurement techniques, and developing new ones, in order to meet ever more demanding technological and industrial challenges. With the expertise that this research brings, we can also help organisations to understand the impact of reliable temperature measurement on their processes, solve their challenging measurement problems and enable innovative improvements in measurement techniques. Find out more about the research that we are doing in this area.
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More information on the SI unit definitions from BIPM